Method for coating electrically conductive particles by grafting a polymer layer

ABSTRACT

The invention concerns a method for forming a coating on conductive particles by grafting a polymer and/or a copolymer of the particles from a bath containing at least a monomer from which the polymer and/or copolymer is formed. The method consists of producing the grafting by electrochemical reduction of the monomer in an electrolytic solution, where at least a cathode and an anode are provided, and containing an aprotic solvent, a support electrolyte and the monomer(s) required for polymerizing or copolymerizing the coating, by suspending the particles and moving the solution so as to produce an intermittent contact between the particles and the cathode, and to form, by polymerization or copolymerization, the coating on the particles by applying an electric potential.

The present invention relates to a process for forming a coating onconductive particles by grafting a polymer and/or copolymer onto theseparticles using a bath containing at least one monomer from which thepolymer and/or copolymer is formed. The expression “conductive particle”applies equally well to powders, granules and short fibres as to objectsof any shape and relatively small size.

The production of composites comprising a polymer matrix in whichelectrically conductive particles are uniformly dispersed is limited atthe present time by the instability of the conductive filler/polymerinterface. To alleviate this drawback, several processes involveprecoating the fillers with an organic component which will ensure, atleast partially, that the particles are bonded to the polymer matrix ofthe composite.

In fact, this approach provides only a very limited solution to thestability of the final composite since the problem of conductiveparticle/polymer matrix adhesion is transferred to the conductiveparticle/coating layer interface.

An improvement to the known processes has in particular been describedin 1995 by N. Tsubokawa and S. Hayashi (J.M.S.—Pure Appl. Chem., 1995,A32, 525-535) which relates to the chemical grafting of polymers onto acarbon powder preoxidized by nitric acid, so as to form OH, COOH and C═Ofunctional groups on the surface. The grafting of the polymer takesplace in two steps: grafting of the functional group, then chemicalbonding of the polymer. The grafting of the polymer is not controllablesince the density of grafted chains is limited by the surface density ofthe functional groups, which is random moreover. The attachment of thepolymer is not really stable since, in some cases, it is thehydrolysable “ester” functional groups which provide thegraphite/polymer bond. Finally, these authors use either monomers whosepolymerization is initiated by the grafted functional group, or alreadyformed polymer chains whose size limits the accessibility to thesubstrate.

One of the essential aims of the present invention is to remedy thedrawbacks of the known coating processes and to present a process makingit possible to form a coating on electrically conductive particles whichadheres perfectly and uniformly, so that these particles thus coated orencapsulated are particularly suitable for the manufacture of compositesfilled with such conductive particles.

The object of the process according to the invention is therefore tocover the particles with a stable polymer layer capable of dispersingthem in a stable and homogeneous manner in a conductive ornon-conductive polymer matrix provided that, of course, measures aretaken to ensure that this matrix and the polymer coating the particlesare miscible, or even mutually reactive.

For this purpose, according to the invention, the grafting is carriedout by electrochemical reduction of the said monomer in an electrolysisbath in which at least one cathode and one anode are provided and whichcontains an aprotic solvent, an electrolyte support and the monomer ormonomers required to form, by polymerization or copolymerization, thecoating, by putting the particles into suspension in the abovementionedbath and by making them move therein, so as to create intermittentcontact between the said particles and the cathode, and to form, bypolymerization or copolymerization, the coating on the particles byapplying a potential or a current which drives the reaction system intoa passivation region corresponding to the inhibition peak detected byvoltammetry, the cathode remaining insensitive to the saidpolymerization or copolymerization during application of theabovementioned potential or current.

Further details and features of the invention will emerge from thedescription given below, by way of non-limiting examples, of a fewparticular ways of implementing the process according to the invention,with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, by way of example, the passivation region on a typicalvoltammogram of a cathode of the same kind as that of the particles tobe covered.

FIG. 2 is a schematic elevation of an electrochemical cell allowing theprocess according to the invention to be carried out, the electrodes inthe cell having been omitted.

FIG. 3 is a top view of this same cell.

FIG. 4 is a vertical partial section, on a larger scale, of twoelectrodes fitted into the cell of the previous figures.

FIG. 5 is a front view of a plate forming the working cathode in theembodiment according to FIG. 4.

FIG. 6 is a graph showing an endotherm specific to the cyclization ofpolyacrylonitrile on a graphite carbon powder.

FIG. 7 is a graph showing the percentage by weight of polymer graftedonto carbon black as a function of the electrolysis time.

FIG. 8 is a similar graph to that in FIG. 7, relating to two differenttypes of carbon particle.

In the various figures, the same reference numbers relate to the sameelements.

In general, the process according to the invention differs from theprior art by the combination of the following points:

direct electrochemical grafting of the polymer onto the conductivesubstrate, that is to say with no intermediate link;

the grafting takes place only under strict control of the appliedpotential;

intermittent contact between the particles to be covered and the workingelectrode (cathode), which remains inert with respect to the grafting.

In fact, the proposed process is characterized by electrografting of amonomer unit which then acts as a polymerization-initiating entity.

More particularly, according to the invention, it has proved possible,in an unpredictable way given the prior art relating to the coating ofconductive particles with a polymer, to obtain such a perfectly stableand homogeneous coating in an electrolysis bath containing the monomer,an aprotic solvent and an electrolyte support, by electrochemicalreduction of this monomer, induced during intermittent contact betweenthe said particles and a cathode placed in this bath, as long as awell-defined potential or current is applied to the cathode and as longas the latter remains insensitive to the polymerization of the monomerduring application of this well-defined potential or current.

This is because, as shown in FIG. 1, before the reduction of the solventor of the cation of the electrolyte support, two separate potential andcurrent peaks are observed, namely a peak I and a peak II. It has beenfound that grafting and polymerization of the monomer at the surface ofthe conductive particles in contact with the cathode occur around peakI. If the potential applied to the cathode does not exceed this value,it has been found, unpredictably, that these particles are effectivelycovered with a thin film of grafted polymer. If the particles are formedfrom carbon, there will therefore be continuity between the carbon andthe polymer.

Thus, the electrolysis of a solution formed from acrylonitrile in anaprotic solvent (acetonitrile) in the presence of an electrolyte support(Et₄NClO₄) results in a film of polyacrylonitrile (PAN) being graftedonto the conductive particles, formed for example from carbon. The PANfilm formed under the electrolysis conditions defined above is insolublein dimethylformamide (DMF) which is a good solvent for PAN. On the otherhand, around the second peak II, a thick PAN film forms which iscompletely soluble in DMF.

Application of the process according to the invention therefore requiresrefined control of the electrolysis conditions: it is necessary to workonly in the region of peak I (the inhibition peak) in order to obtainhomogeneous grafting of the polymer onto the cathode. In practice, thiscondition can be achieved using two techniques, namely potentiostaticelectrolysis, in which the cathode potential is set, with respect to areference electrode, in the region of peak I and galvanostaticelectrolysis, in which the polymerization current is set to a valuebelow the maximum current (ip) defined in voltammetry. The currentdensity is adjusted depending on the mean diameter of the particles tobe coated and on the dynamic stirring conditions, so as not to exceed acurrent corresponding to the first peak I with respect to the fixedelectrode.

The originality of the process is the formation of intermittent contactsbetween conductive particles to be covered and a fixed cathode chosenfrom a conductive material onto which it is impossible to graft apolymer using the process according to the invention. Withoutrestriction, zinc, tin or even silicon carbide (SiC) may be used as thecathode in the process. Intermittent contact is provided by bringing theparticles into transient contact with the cathode either by the usualprocess of stirring a liquid, such as one using a propeller stirrer, amagnetic stirrer, or an ultrasonic field, or by a mechanical movement ofthe cathode, such as rotation, oscillation or vibration. It is alsopossible to use an enveloping cathode, able to serve as the electrolysistank and itself undergoing a movement. Under the dynamic stirringconditions, each particle statistically comes into contact with theworking electrode in renewed spatial orientations. At each contact, themonomer is polarized and an electron transfer contact with the electrodeis transiently established: this step is the necessary and sufficientcondition for the polymerization to be able to start. After thisinitiation, the stirring returns the particle to the solution, where thepolymerization will continue chemically since the monomer is availablein the solution. During this polymerization step, the polarization ofthe cathode no longer has any effect around the particle. The latter hasnot necessarily been covered in its entirety during the first contactwith the cathode, but it will be able to be so during subsequentcontacts between the uncovered parts of the particle and the cathode,which still remains polarized under the potential conditions specifiedabove.

According to the invention, several monomers are able to be grafted atthe first peak, but the monomer/solvent pair must be chosen judiciously.By way of non-limiting example, mention should be made of the followingsystems: acrylonitrile/acetonitrile, acrylonitrile/dimethylacetamide,acrylonitrile/N,N-dimethylformamide, acrylonitrile/pyridine, ethylacrylate/N,N-dimethylformamide, ethyl acrylate/pyridine,2-trimethylsilyloxyethyl methacrylate/N,N-dimethylformamide, methylmethacrylate/N,N-dimethylformamide, tert-butylacrylate/N,N-dimethylformamide, n-butyl acrylate/N,N-dimethylformamide,allyl methacrylate/N,N-dimethylformamide and glycidylmethacrylate/N,N-dimethylformamide. All these solvents belong to theclass of aprotic solvents in which grafting is possible. The choice ofelectrolyte support has, in principle, no fundamental influence on thegrafting, but it is preferable to use an electrolyte support which doesnot bring residual water from the solvent into the double layer of thecoating formed on the particles.

The shape and the volume of the electrolysis cell used for carrying outthe process according to the invention are determined by the nature ofthe particles to be covered and by the choice of the type ofintermittent particle/cathode contact. The electrolysis cell, asillustrated in FIGS. 2 to 4, comprises a vessel 1 containing a solutionformed from a solvent, from a monomer or from a mixture of comonomers,from an electrolyte and from conductive particles in suspension.Provided in the bottom of this vessel 1 is a bar magnet 2, which rotatesabout a vertical spindle, thus ensuring that the solution is homogeneousand that the conductive particles to be coated with a polymer are set inmotion.

The upper part of this vessel 1 has three internally ground conicalhousings 3, 4 and 5 from which electrodes, for example a central workingelectrode 6, a counterelectrode 8 and a reference electrode 8′, aresuspended.

These electrodes are shown in greater detail in FIG. 4. The workingelectrode 6 includes, at its free end, a small plate (50×50 mm with athickness of 0.5 mm), for example made of zinc, which is perforated asshown in detail in FIG. 5. The number of holes and their diameter arenot critical as long as there is free circulation of the suspension andthe particle/zinc contact time is long enough to allow the particles tobe polarized to the potential applied to the cathode. This plate 7 issuspended by a clip 9 from a conductor 10 which extends inside, alongthe spindle, of a glass sleeve 11, the upper opening 12 of which isclosed by an epoxy adhesive plug 13. The sleeve 11 has a ground conicalouter wall 14 which fits in a substantially sealed manner in the housing4 in the vessel 1.

The reference electrode 8′ and the counterelectrode 8 have the sameconstruction and comprise a platinum foil wound in the form of acylinder 15 having an area of the order of 10 cm². This cylinder 15 issuspended from a conductor 16 which extends along the spindle of a glasssleeve 17 closed at its upper end 18 by an epoxy adhesive plug 19.

As in the case of the electrode 6, the electrodes 8 and 8′ have aconical outer wall 20 which fits in a sealed manner into thecorresponding housings 3 and 5 in the vessel 1.

In a variant, the working electrode 7 may, for example, be formed by abar of silicon carbide, for example 2 mm in diameter and 2 cm in length,or else by a bar of tin.

The counterelectrode providing the flow of current must be inert: it ismade, for example, of platinum or of glassy carbon. If electrolysis inpotentiostatic mode is preferred, the reference electrode is either astandard electrode for organic media (Ag/AgClO₄/solvent) or a platinumpseudo-reference electrode.

The manipulations are carried out in an atmosphere protected from oxygenand from water, the atmosphere having, for example, a water content ofabout 5 to 50 ppm.

The specific illustrative examples given below, which apply to anelectrolysis cell of the type shown in FIGS. 2 to 5 and described above,allow the process according to the invention to be illustrated further.

EXAMPLE 1 Grafting of Polyethyl Acrylate (PEA) onto Graphite CarbonPowder

In this example, the grafting and the electropolymerization of ethylacrylate (EA) are carried out on the surface of graphite particles.Electrical contact is provided by a silicon carbide (SiC) electrode. Theelectrochemical technique used is chronopotentiometry.

The electrolysis bath is based on the following constituents:

150 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.9 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

37.5 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

0.35 g of graphite carbon powder.

An SiC bar (the cathode), a counterelectrode and a pseudo-referenceelectrode (both the latter being made of platinum) are immersed in theelectrolysis solution. A constant current of −15 μA with respect to thepseudo-reference electrode is applied to the SiC bar for 90 minutes. Thecarbon is polarized by stirring the dispersion, using a magneticstirrer. After electrolysis, the carbon is separated by centrifuging andthen washed several times with a solvent for the polymer, so as todissolve any polymer that was not grafted. The presence of grafted PEAis demonstrated macroscopically by the agglomeration of polymer-coatedparticles even after many extractions with a good solvent for PEA.

EXAMPLE 2 Grafting Polyethyl Acrylate (PEA) onto Graphite Carbon Powder

In this example, the grafting and the electropolymerization of ethylacrylate (EA) are also carried out on the surface of graphite particles.Electrical contact is provided by a zinc (Zn) electrode. Theelectrochemical technique used is chronoamperometry. The electrolysisbath is based on the following constituents:

150 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.9 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

40 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

0.2 g of graphite carbon powder.

A 5 cm×5 cm Zn plate (the cathode), a counterelectrode and apseudo-reference electrode (both the latter being made of platinum) areimmersed in the electrolysis solution. A constant potential of −1.7 Vwith respect to the pseudo-reference electrode is applied to the Znelectrode for 21 hours; this potential corresponds to the potential ofthe first peak. The carbon is polarized by stirring the solution. Thepresence of PEA is demonstrated macroscopically, as described in thefirst example. The percentage by weight of polymer grafted onto thepowder is determined by TGA (thermal gravimetric analysis) andrepresents 28% of the total weight of the composite.

EXAMPLE 3 Grafting of Polyacrylonitrile (PAN) onto Graphite CarbonPowder

In this example, the grafting and the electropolymerization ofacrylonitrile (AN) are also carried out on the surface of graphiteparticles. Electrical contact is provided by a 50×50 mm zinc (Zn)electrode pierced with holes 2 mm in diameter. The electrochemicaltechnique used is chronoamperometry. The electrolysis bath is based onthe following constituents:

225 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

2.8 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

35 ml of AN dried over CaH₂ for 48 hours and then distilled underreduced pressure;

1.6 g of graphite carbon powder.

A constant potential of −1.7 V with respect to the pseudo-referenceelectrode is applied to the Zn electrode for 21 hours. The carbon ispolarized by stirring the solution. Next, the carbon is washed severaltimes with DMF, a solvent for PAN, so as to extract the PAN that was notgrafted. The presence of PAN is then demonstrated by DSC (differentialscanning calorimetry) in the form of an endotherm specific to thecyclization of PAN (FIG. 6). The percentage by weight of the graftedpolymer, determined by TGA (thermal gravimetric analysis), is 15% withrespect to the total weight of the composite.

EXAMPLE 4 Grafting of Polyacrylonitrile (PAN) and of Polyethyl Acrylate(PEA) onto Carbon Black

In this example, the kinetics of grafting PAN and PEA onto the surfaceof the carbon black are compared. Electrical contact is provided by azinc (Zn) electrode pierced with holes 2 mm in diameter. Theelectrochemical technique used is chronoamperometry. The electrolysisbaths are based on the following constituents:

1) AN bath:

100 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.15 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

15 ml of AN dried over CaH₂ for 48 hours and then distilled underreduced pressure;

2.25 g of XE2-type carbon black (from Degussa);

2) EA bath:

100 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.15 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80°C. for 24hours;

27 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

2 g of XE2-type carbon black (from Degussa).

A 5 cm×5 cm Zn plate pierced over its entire area with holes 2 mm indiameter (the cathode), a counterelectrode and a pseudo-referenceelectrode (both the latter being made of platinum) are immersed in theelectrolysis solutions. A constant potential of −2.2 V (first peak) withrespect to the pseudo-reference electrode is applied to the Zn electrodefor 15, 30, 60 and 100 minutes in the AN bath, and a potential of −1.7 V(first peak) with respect to the pseudo-reference electrode is appliedto the Zn electrode for 15, 30, 60 and 120 minutes in the EA bath. Thecarbon is polarized by stirring the solution. The various carbonspecimens are then washed several times with DMF, a solvent for PAN andfor PEA, so as to extract any non-grafted polymer. The grafted PEAspecimen is also stripped of any non-grafted PEA by extraction using THFin a Soxhlet apparatus. The percentage by weight of the grafted polymersis determined by TGA (thermal gravimetric analysis). FIG. 7 shows thepercentage by weight of polymer grafted onto carbon black as a functionof the electrolysis time.

EXAMPLE 5 Grafting of Polyethyl Acrylate (PEA) onto Carbon Black andonto Graphite Carbon Powder

In this example, the kinetics of grafting PEA onto the surface of thecarbon black and onto the graphite carbon powder are compared.Electrical contact is provided by a zinc (Zn) electrode pierced withholes 2 mm in diameter. The electrochemical technique used ischronoamperometry. The electrolysis baths are based on the followingconstituents:

1) Carbon black bath:

100 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.15 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

27 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

2 g of carbon black of the XE2-type (from Degussa);

2) Graphite carbon powder bath:

110 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

1.27 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80°C. for 24hours;

30 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

5.2 g of graphite carbon powder.

A 5 cm×5 cm Zn plate pierced over its entire area with holes 2 mm indiameter (the cathode), a counterelectrode and a pseudo-referenceelectrode (both the latter being made of platinum) are immersed in theelectrolysis solutions. A constant potential of −1.7 V (first peak) withrespect to the pseudo-reference electrode is applied to the Zn electrodefor 15, 30, 60 and 120 minutes in each of the electrolysis baths. Bothtypes of carbon particle are polarized by stirring the solutions. Thevarious carbon specimens are then washed several times with DMF and withTHF, both solvents for PEA, so as to extract the polymer which was notgrafted. The percentage by weight of grafted polymers is determined byTGA (thermal gravimetric analysis). FIG. 8 shows the percentage byweight of polymer grafted onto the carbon black and onto the carbonpowder as a function of the electrolysis time. The influence of thenature of the carbon particles on the grafting rate should be noted.

EXAMPLE 6 Grafting of Polyacrylonitrile (PAN) onto Metal Powder

In this example, the grafting of PAN is carried out on the surface of anickel metal powder. Electrical contact is provided by a zinc (Zn)electrode pierced with holes 2 mm in diameter. The electrochemicaltechnique used is chronoamperometry. The electrolysis bath is based onthe following constituents:

108 ml of DMF dried over P₂O⁵for 72 hours and then distilled underreduced pressure;

1.24 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

17 ml of AN dried over CaH₂ for 48 hours and then distilled underreduced pressure;

5.12 g of nickel particles (INCO, type 110).

A constant potential of −1.7 V (the potential of the first peak), withrespect to the pseudo-reference electrode, is applied to the Znelectrode for 30 minutes. The metal particles are polarized by stirringthe solution. The carbon is then washed several times with DMF, asolvent for PAN, so as to extract the non-grafted PAN. The percentage byweight of grafted polymer, determined by TGA (thermal gravimetricanalysis), is 10%.

EXAMPLE 7 Grafting of Polyethyl Acrylate (PEA) onto Short Carbon Fibres

In this example, the grafting and the electropolymerization of ethylacrylate (EA) are carried out on short carbon fibres, the latter not bbeing directly polarized by permanent contact. Electrical contact isprovided by a zinc (Zn) electrode pierced with holes 2 mm in diameter.The electrochemical technique used is chronoamperometry. Theelectrolysis bath is based on the following constituents:

200 ml of DMF dried over P₂O₅ for 72 hours and then distilled underreduced pressure;

2.3 g of Et₄NClO₄ (electrolyte support) dried in vacuo at 80° C. for 24hours;

60 ml of EA dried over CaH₂ for 48 hours and then distilled underreduced pressure;

0.1 g of short (2 to 3 mm) carbon fibres of the AS-4 type (fromHercules).

A 5 cm×5 cm Zn plate (the cathode), a counterelectrode and apseudo-reference electrode (both the latter being made of platinum) areimmersed in the electrolysis solution. A constant potential of −1.7 V(the potential of the first peak) with respect to the pseudo-referenceelectrode is applied to the Zn electrode for 21 hours. The short carbonfibres are polarized by stirring the solution. The presence of PEA isdemonstrated macroscopically by the sticky nature of the polymer, evenafter repeated extraction using a good solvent for the polymer, and alsomicroscopically by SEM (scanning electron microscopy). The percentage byweight of the polymer is determined by TGA (thermal gravimetricanalysis); a value of 52% is obtained.

Of course, the invention is not limited to the embodiments describedabove, and many other variants may be envisaged without departing fromthe scope of the present invention both with regard to the nature of theconductive particles to be coated with a polymer film and with regard tothe electrolysis cell to be used.

What is claimed is:
 1. A process for forming a coating on conductiveparticles, said process comprising the steps of: grafting at least onematerial selected from a group consisting of a polymer and a copolymeronto these particles by using a bath containing at least one monomerfrom which the polymer and copolymer are formed; carrying out thisgrafting by electrochemical reduction of the said monomer in anelectrolysis bath containing an aprotic solvent, an electrolyte support,and the monomer required to form the coating, said electrolysis bathalso containing at least one cathode and one anode, putting theparticles into suspension in the bath and making them move thereinresponsive to a current in said electrolysis bath, said movementcreating intermittent contact between said particles and the cathode,and coating by polymerization on the particles being formed by applyinga potential which drives an electrodialysis reaction into a passivationregion corresponding to an inhibition peak detected by voltammetry, thecathode remaining insensitive to the polymerization during theapplication of the potential.
 2. The process according to claim 1,further comprising a use of carbon particles selected from a groupconsisting of granules, powder and fibres.
 3. The process according toclaim 1, further comprising a use of metal particles selected from agroup consisting of powder, granules and fibres.
 4. The processaccording to claim 1, wherein the cathode is made of a conductivematerial onto which it is impossible to graft the polymer, saidconductive material being selected from a group consisting of zinc, tinor silicon carbide.
 5. The process according to claim 1, wherein amonomer/aprotic solvent pair is formed from a material selected from agroup consisting of acrylonitrile/acetonitrile,acrylonitrile/dimethylacetamide, acrylonitrile/N,N-dimethylformamide,acrylonitrile/pyridine, ethyl acrylate/N,N-dimethylformamide, ehtylacrylate/pyridine, 2-trimethylsilyloxyethylmethacrylate/N,N-dimethyulformamide, methylmethacrylate/N,N-dimethylformamide, tert-butylacrylate/N,N-dimethuylformamide, n-butyl acrylate/N,N-dimethylformamide,allyl methacrylate/N,N-dimethylformamide and glycidylmethacrylate/N,N-dimethylformamide.
 6. The process according to claim 1,further comprising the steps of providing a reference electrode; settinga cathode potential with respect to a reference electrode, in the regionof an inhibition peak in order to maintain conditions for electrolysisnear an inhibition peak.
 7. The process according to claim 1, furthercomprising the steps of setting the polymerization current to a valuebelow a maximum current and keeping an approximately constant currentpulse below a current for causing polymerization at a first inhibitionpeak, so as to avoid a second polymerization mechanism.
 8. The processaccording to claim 1, and the further step of applying the electrolysiscurrent continuously.
 9. The process according to claim 1 and thefurther step of applying the electrolysis current intermittently.